Changes to the Digital Millennium Copyright Act (DMCA) which come into force tomorrow, Saturday the 26th of January will make it illegal to unlock a cellphone in the US and use it with another mobile carrier.
So basically as of tomorrow if you unlock your cellphone to use on another network in the US, you will apparently be breaking the law, and you will have to ask your carrier permission to have the device unlocked.
This is not the same as jailbreaking you device to run modified software, this will still be legal, it is just the unlocking off the device without the carriers permission that will now be made illegal from tomorrow.
Source Mashable